Liquid water and an atmosphere are necessary for a planet to possibly sustain life, even if it might not be a great place to live, the scientists say.

The scientists determined that the planet, which they have called Gliese 581g, has a mass three to four times that of Earth and an orbital period of just under 37 days.

Its mass indicates that it is probably a rocky planet and has enough gravity to hold on to an atmosphere, according to Dr Steven Vogt, professor of astronomy and astrophysics at the University of California, Santa Cruz, and one of the leaders of the team that discovered the planet.

If Gliese 581g has a rocky composition similar to Earth's, its diameter would be about 1.2 to 1.4 times that of the Earth, the researchers say.

The surface gravity would be about the same or slightly higher than Earth's, so that a person could easily walk upright on the planet, Vogt says.

Gliese 581g was discovered by scientists working on the Lick-Carnegie Exoplanet Survey, during 11 years of observing the red dwarf star Gliese 581, which is only 20 light years from Earth.

For astronomers, eleven years of observation is considered a short time and 20 light years, which is roughly 117.5 trillion miles, rather close. The Sun is around eight and a half light minutes from Earth.

"The fact that we were able to detect this planet so quickly and so nearby tells us that planets like this must be really common," says Vogt.

Temperature variations

The planet is tidally locked to its star, meaning that one side is always facing the star and basking in perpetual daylight, and the other is in perpetual darkness because it faces away from the star.

With surface temperatures decreasing the further one goes toward the dark side of the planet and increasing as one goes into the light side, the most habitable part of the new planet would be the line between darkness and light, which is known as the "terminator".

The researchers estimate that the average surface temperature of the planet would be between -31°C to -12°C.

But actual temperatures would range from "blazing hot on the side facing the star to freezing cold on the dark side," they say.

The findings, which will be published in The Astrophysical Journal and posted online at the pre-press website arXiv.org , "offer a very compelling case for a potentially habitable planet," says Vogt.

"Any emerging life forms on the new planet would have a wide range of stable climates to choose from and to evolve around, depending on their longitude."

'Goldilocks' find

In their report, the scientists announce the discovery of two new planets around Gliese 581, bringing the total number of known planets around this star to six.

That is the most yet discovered in a planetary system other than Earth's solar system.

Like planets in Earth's solar system, the planets around Gliese 581 have nearly circular orbits.

Two previously detected planets around Gliese lie at the edges of the habitable zone, one on the hot side and one on the cold side of the star, and are probably not habitable.

The newly discovered planet g, however, lies right in the middle of the habitable zone.

"We had planets on both sides of the habitable zone - one too hot and one too cold - and now we have one in the middle that's just right," Vogt said, recalling the porridge that Goldilocks found in the children's story "The Three Bears."